I am only posting this because this has happened a few times and it always takes me a moment to remember how to fix it. The moment is usually right after I download and install the latest drivers, reboot and realize it is still broken.
If you notice any of the following odd behavior:
You can check to see if your stylus is in calibration. To do this open the Wacom Tablet Properties > About > Diagnostics page
Look at “Pressure”, it should read 0% until the stylus touches the tablet and go back to 0% as soon as you pull away. If it is reading 7~8% with no contact it is enough to keep other buttons from working correctly and drive you crazy.
Try pulling the tip out of your stylus and putting it back in.
Now check your pressure again, you should be good to go.